FIG. 1 illustrates a prior art system 10 of how a transmitter 20 and a receiver 30 communicate. The transmitter 20 can be, for example, a computer, a DVD player or other video source and the receiver 30 can be a monitor or a television. Communication between the transmitter 20 and the receiver 30 is typically achieved via the DVI (digital visual interface) 40 which is a single interface that contains various lines bundled into one cable. An HDMI link (high definition multi-media interface) or some other variant can also achieve communication between the transmitter 20 and the receiver 30. Some of these lines include a hot plug signal, a power line, TMDS® (transition minimized differential signaling) 50 (typically used for carrying video data) and a DDC (display data channel) bus 60.
The DDC bus 60 is a serial 2-wire interface that has one data line and one clock line. This serial protocol is believed to have been developed, at least in part by the Philips Corporation of Holland. Philips part #PCA9515 is an integrated circuit which implements the so-called I2C bus. One of the primary purposes of the DDC bus 60, when used as an I2C bus, is to read an EDID PROM 70 (extended data interface device programmable read only memory) which includes data concerning the receiver 30. The DDC bus 60 can also be used for data transfer with HDCP 80 (high-bandwidth digital content protection), which is an encryption device that provides content protection.
Several problems are associated with the DDC bus 60 that make it undesirable for certain applications. One problem is that it can not be of an extended physical length due to electrical issues such as overwhelmed capacitive load budgets and transmission line effects that degrade signal timing parameters. Another problem is that its data carrying capacity is limited to about 400 kilobits/second. The data on the DDC bus 60 can be easily eavesdropped and even manipulated and since it may connect to several devices in the transmitter 20, security is also an issue. Finally, any attempt to solve these problems would need to take into consideration legacy issues for the purpose of backward compatibility. That is, the DDC bus is very widely used and any attempt to improve upon it would need to be compatible with transmitters and receivers that use the standard DDC bus interface.
Accordingly, what is needed is a method and apparatus for a DDC compatible two-wire serial command interface which allows for high speed data transmission, extended cable length, data security and still provide backward compatibility.